In a wireless transmission system including a plurality of terminals and at least one base station having functions such as a handover function, types of information uplink-transmitted by a terminal are classified into traffic information and control information. The traffic information includes user data, and the control information includes information on system operation, base station scheduling, automatic request (ARQ), multiple input multiple output (MIMO), adaptive modulation and coding (AMC), power control, uplink synchronization, terminal resource allocation request, and random access functions.
The amount of resources required to transmit the uplink control information by the respective terminals is less than that required to transmit the traffic information, but types of control information are various and they are frequently transmitted. Therefore, to transmit the control information to the base station, a considerable amount of bandwidth and complicated processes are required. Accordingly, a method for transmitting the control information and a method for allocating resources for the control information transmission are required to be differently designed from the traffic information transmission, and a method for selecting the control information transmission method by the terminal may considerably affect system performance.
The ranging information among the control information is used to adjust uplink time synchronization and power. Six subchannels form a basic unit when the base station allocates the resources for transmitting the ranging information by using a partial usage subchannel (PUSC) method (one of OFDMA resource allocation methods by the IEEE 802.16), and 8 subchannels form the basic unit when the base station allocates the resources for transmitting the ranging information by using an optional PUSC method. In the PUSC, a subchannel is formed by randomly selecting 6 tile configurations from the entire band, in which one tile configuration is defined by four neighboring sub-carriers and three neighboring symbols. In the optional PUSC, a sub channel is formed by randomly selecting 8 tile configurations, in which one tile configuration is formed by three neighboring subcarriers and three neighboring symbols. Accordingly, the basic unit for transmitting ranging information includes 144 subcarriers defined in 3 symbols.
Among the three symbols, the first two symbols are used to transmit an initial ranging signal or a handover ranging signal, and the other symbol is used to transmit a periodic ranging signal or a band request ranging signal.
Since the terminal transmits a pseudo random noise (PN) code with a length of 144 subcarriers to perform an initial ranging or handover ranging operation by using a channel allocated by the base station, the base station may discern pieces of ranging information when the pieces of ranging information are simultaneously transmitted by using the same channel.
In addition, since the initial ranging or handover ranging signal is transmitted while uplink synchronization is not yet obtained, the first symbol uses a cyclic prefix and the second symbol uses a cyclic postfix to sequentially transmit the same code. Further, since the periodic ranging or band request ranging signal is transmitted while the uplink synchronization is obtained, the signal is transmitted by using 144 subcarriers of the third symbol among the symbols allocated to perform the ranging operation by the base station.
Uplink ACK/NACK information among the control information is used to perform an ARQ operation for a downlink (DL) packet or a hybrid automatic repeat request (H-ARQ) operation. The terminal uses the tile configuration allocated in the optional PUSC method by the base station to uplink-transmit the ACK or NACK signal.
FIG. 1 is a diagram representing a conventional ACK/NACK code configuration method.
Referring to FIG. 1, ACK/NACK codes are formed in code columns perpendicularly crossing each other, and an ACK or NACK state is determined by correlation.
Uplink channel sounding information among the control information is defined to support closed-loop MIMO. When a transmitter (generally, a transmitter at the base station) knows channel information (i.e., downlink channel information when the transmitter is at the base station), frequency usage efficiency may be increased by using a precoding method. There are two methods for providing the downlink information to the base station by the terminal.
In one of the two methods, a common preamble or pilot signal is transmitted so that the base station may discern transmitting antennas, and the terminal uses it to measure a downlink channel and uses an uplink control channel to report the downlink channel to the base station. In the other of the two methods, the terminal transmits the uplink channel sounding information, the base station uses it to measure the uplink channel, and the base station acknowledges the downlink channel by using a reversible channel characteristic in a time division duplex (TDD) method. In the IEEE 802.16 OFDMA, a predetermined number of uplink symbols are used to transmit the channel sounding signal.
FIG. 2 is a diagram representing a frame configuration and an uplink control channel configuration in a conventional OFDMA TDD system.
Referring to FIG. 2, the horizontal axis indicates times divided by OFDM symbols and the vertical axis indicates frequencies divided by subchannels. Various pieces of uplink control information required to operate a channel quality indicator and the MIMO are defined to be transmitted by using fast feedback resources.
FIG. 3 is a diagram representing inter-carrier interference and inter-symbol interference generated on the ranging channel and the ACK/NACK channel when the initial and handover ranging operations are performed according to IEEE 802.16.
When the terminal uses the first three OFDM symbols of an uplink frame to transmit the ranging information and the ACK/NACK information, the information is transmitted without obtaining the uplink synchronization, and periodic ranging, band request ranging, and ACK/NACK control signals are transmitted while the uplink synchronization is obtained.
Referring to FIG. 2, the same subcarrier is allocated to transmit the initial ranging, handover ranging, periodic ranging, and band request ranging information. Therefore, in FIG. 3, since the initial ranging or handover ranging signal in sections t5 to t7 of the third OFDM symbol affects the periodic ranging or band request ranging signal transmitted in the third symbol as the ISI, periodic ranging or band request ranging performance is deteriorated. In addition, since the periodic ranging signal or band request ranging signal in the section t5 to t7 of the third OFDM symbol affects the initial ranging signal or handover ranging signal transmitted in the second symbol as the ISI, receiving performance of the initial ranging signal or handover ranging signal is deteriorated.
Further, sudden changes of the initial ranging and handover ranging signals generated in the sections t1, t2, t6, and t7 cause a spectral emission effect, which affects the ranging signal and the ACK/NACK signal as the ICI, and therefore receiving performance is deteriorated. The sudden changes of the periodic ranging signal or band request ranging signal and the ACK/NACK signal in the section t3 and t4 cause the spectral emission effect, which affects the initial ranging and handover ranging signals as the ICI, and therefore the receiving performance is deteriorated.
In the above uplink channel allocation method, since the uplink channel sounding signal is transmitted by using one entire OFDM symbol as shown in FIG. 2 when the uplink channel is allocated to one user, resources may be problematically wasted.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.